I got one of those inexpensive AD9850 DDS modules from eBay and had a go at building a simple controller for it. I wanted to adjust the frequency from 0 to 60 MHz only using a rotary encoder with a push-switch. There have been a few iterations so far, based on different microcontrollers, including an Arduino Nano, and even an ATTiny2313. Details here!

The HC-SR04 Ultrasonic Sensor on a Tiny13

Here is a tutorial of sorts based on the popular and inexpensive HC-SR04 Ultrasonic Sensor running off of an Atmel ATTiny13 in AVR assembly language. From there it's a no-brainer to get it going on any of the Atmel AVR series microcontrollers. Check it out!